Mechanical response of the spine to various dynamic loading conditions can
be analyzed by way of in vitro and in vivo studies. Ethical concerns,
interpretation of conclusions reached in animal studies, and lack of detailed
stress distributions in the disc components are the major disadvantages of
relying solely on in vivo studies. Intraspecimen variability, difficulty in
including muscle activity, and inability to mimic fluid exchange into the disc
during unloading are some of the disadvantages of in vitro models. The
poroelastic finite element models can provide a method of understanding the
relationship between biomechanical performance of the disc due to cyclic
loading and disc degeneration. A poroelastic finite element model, including
regional variation of strain-dependent permeability and osmotic pressure, was
used to study the effect of disc degeneration on biomechanical properties as
well as propagation of failure in the disc components when cyclic loading was
applied to the lumbar disc. The results predicted that healthy discs were much
more flexible than degenerated discs, and the disc stiffness decreased with
increasing the number of load cycles independent of degenerative condition.
Failure was found to progress as the drained elastic properties of the disc
components decreased due to the presence of failure.
Poroelastic finite element modeling, including strain-dependent
permeability and osmotic pressure, is the most advanced analytical tool
currently available that can be used to understand how cyclic loading affects
the biomechanical characteristics of a degenerated lumbar disc. However, a
complete understanding of behavior of the intervertebral disc will ultimately
be achieved only with use of a combination of computational models together
with in vitro and in vivo experimental methods.
Finite element models of discs with varying degrees of disc degeneration
will help clinicians understand the initiation and progression of disc failure
and degeneration and will assist in the development of approaches to stimulate
the regeneration of disc tissues.